Stare decisis is a Latin legal maxim that roughly translates to "to stand by decided matters". In the law, it means that cases that are alike should be decided alike. It is a key concept behind the common law and its dependence on precedent;
The judge's decison contains the ratio decidendi (the reasons for the decision), and obiter dictum (discussion of the law not directly relevant to the decision). As a rule, future judges are bound by the "ratio", but not the "obiter".
A trial judge who ignorse precedent is likely to be reversed by a higher court. However, the highest courts are not bound by their own precedent, although they are more likely to apply it. It is rare, but high courts do reverse the prior precedent on a matter when it turns out to be unworkable or unfair.
In modern law, statutes are the most common way a legislature will deal with perceived faults in the common law as legislation on a matter overrules court precedent. However, cases decided before the statute is passed may still be referred to in order to resolve any ambiguities in the language of the legislation. If a proposed interpretation of ambiguous statutory language would have the effect of not changing the common law, courts will generally reject that interpretation as unreasonable.